This invention relates generally to magnetic tape transports, particularly to those suitable for use as or in a subsystem designed to enable a host system to obtain access to data on magnetic tape. More particularly, the invention relates to a method of, and apparatus for, controlling the gain of a read amplifier, included in such tape transports, by writing and reading a reference pattern, known as a reference burst, on the tape in the face of dust that may accumulate on the tape or on the transducer.
Magnetic tape in cartridge form is being widely used as an external storage media of computer systems. Tape cartridges are now available in several different forms. Examples are the two-reel, belt driven cartridge introduced by Minnesota Mining and Manufacturing Company, and the "digital cassette" based on the standard audio cassette developed by Philips and made to similar dimensions though with more precision.
Such tape cartridges include a length of magnetic tape with a plurality or multiplicity of parallel tracks extending longitudinally of the tape. The tape is bidirectional, so that the tracks consist of forward tracks, on which data is written and read during tape, travel in a predetermined forward direction, and reverse tracks.
Usually, in recording data on such bidirectional, multitrack tape, an alternating current signal known as a reference burst is written and read immediately before writing data on the first of forward tracks and on the first of reverse tracks. The output from either of the forward and reverse read heads is directed through a read amplifier into a peak hold circuit. The peak value of either of the reference bursts, held by the peak hold circuit, is then digitized and sent into the controller of the tape transport. The controller controls the gain of the read amplifier so as to approximate the peak of the recovered reference burst to a predetermined value stored therein.
In tape transports having two separate heads for reading and writing data on the tape traveling either forwardly or reversely, data is usually written by what is known as a read-while-write check method. The method is such that data is read and checked for errors as soon as it has been recorded. The gain of the read amplifier is held at a value that has been determined as above, while data is being written by this method. For reading prerecorded data, on the other hand, the gain of the read amplifier is controlled by recovering the prerecorded reference bursts. The reference bursts are additionally utilized for positioning the heads on the tape tracks.
A problem has been encountered in writing and reading reference bursts on the tape preparatory to the recording of data, particularly in cases where the tape transports are used in a dusty environment. Dust on the tape or the heads can prevent the writing and reading of reference bursts, making impossible the writing of data.
Reference bursts are particularly susceptible to the effects of dust because the tape is usually alternately run forwardly and reversely. Such alternate bidirectional travel of the tape results in the accumulation of dust on its opposite end portions as the tape slides past the heads. The accumulated dust can cause errors in writing and reading reference bursts on the end portions of the tape.